Polyurethane resins, which are typical of thermosetting resins, are relatively inexpensive and are easy to mold, and the foamed products thereof are widely used over the entire range of articles of daily use, including automotive parts. However, polyurethane resins are flammable, and once they ignite, they have a major drawback of carrying out uncontrollable combustion. Various efforts have thus been made to produce flame-retardant polyurethane foam. Nowadays, flame retardance is legally compulsory in some fields featuring the use of polyurethane, such as automotive interiors.
Generally, in order to impart flame retardancy to polyurethane, the method of adding a halogen-containing phosphate ester as a flame retardant is adopted. Additive-type flame retardants of halogen-containing monomeric phosphate ester are usually used, such as tris(chloroethyl) phosphate and tris(chloropropyl) phosphate. However, these flame retardants tend to be vaporized at high temperature due to their low molecular weight. When the polyurethane foam containing such a flame retardant is used for automotive sheet and the like, as the temperature inside the automobile increases in summer or under similar high-temperature conditions, a phenomenon of fogging tends to occur in which phosphate components, amine catalysts contained in the starting materials, or salts of amine with hydrogen halides produced by the decomposition of the phosphate components are vaporized or scattered inside the automobile and the glass of the automobile becomes cloudy. Further, the flame retardant property of the foamed product is sometimes impaired. The vaporized or scattered materials also pose the risk of adversely affecting humans.
A method that has thus been proposed in order to reduce the amount of flame retardants which are vaporized is to use additive-type flame retardants of halogen-containing condensed phosphate oligomer. However, monomer components generally remain in an amount of about 5 to 20 wt % in oligomer types of flame retardants, so even when oligomer types of flame retardants are used, the problem of the flame retardant becoming vaporized at elevated temperatures cannot still be overcome due to the presence of such low molecular weight components.
Other known flame retardants are additive-type flame retardants of halogen-containing highly condensed phosphate esters in which the phosphate esters are condensed to a higher degree in order to minimize the amount of residual monomer components. However, such highly condensed phosphate esters have poor workability because of their high viscosity, and tend to decompose, resulting in the formation of hydrogen halide salts of amines, with the risk of scattering and adverse effects on other physical properties such as scorch resistance.
Furthermore, the aforementioned types of flame retardants all contain halogens, and there is concern that their vaporization might affect humans and they produce dioxins when burned.
Although the use of additive-type flame retardants of halogen-free phosphate ester could overcome problems such as the formation of hydrogen halide salts of amines and the production of dioxins when burned, the vaporization of the phosphate esters itself cannot be prevented.
Methods that have thus been studied include preventing flame retardants from being vaporized by using flame retardants with reactive functional groups, referred to as reactive flame retardants, which are incorporated into the resin skeleton of the polyurethane foam by reacting with starting materials.
The polyurethane foam is formed by the reaction between isocyanate groups of a polyisocyanate and two types of hydroxyl groups, i.e. hydroxyl groups in the polyol and hydroxyl groups in water serving as the blowing agent. However, when reactive flame retardants of phosphate ester containing reactive functional groups are used, it is necessary to control the reaction between the isocyanate groups and three different types of reactive functional groups, making it difficult to fully satisfy the intended properties of foamed product in the conventional manner.
Japanese Unexamined Patent Publication No. 2001-11302, for example, describes a method in which a reactive type of phosphate ester containing alcoholic hydroxyl groups is used to produce a flame-retardant flexible polyurethane foam. Japanese Unexamined Patent Publication No. 2001-151919 describes a method in which both a reactive type of phosphate ester containing alcoholic hydroxyl groups and an additive type of oligomeric phosphate ester are used to produce a flame-retardant flexible polyurethane foam with good flame laminate property. In these methods, however, it is difficult to achieve the well-balanced reaction between the three types of hydroxyl groups and the isocyanate groups. Thus, open cells cannot be formed by simply varying the amount of the reactive phosphate ester and a closed cell foam tends to be formed, resulting in the low air permeability and the high compressive strain of the foamed product.